Targeted editing of nucleic acid sequences, for example, the introduction of a specific modification into genomic DNA, is a highly promising approach for the study of gene function and also has the potential to provide new therapies for human genetic diseases.1 An ideal nucleic acid editing technology possesses three characteristics: (1) high efficiency of installing the desired modification; (2) minimal off-target activity; and (3) the ability to be programmed to edit precisely any site in a given nucleic acid, e.g., any site within the human genome.2 Current genome engineering tools, including engineered zinc finger nucleases (ZFNs),3 transcription activator like effector nucleases (TALENs),4 and most recently, the RNA-guided DNA endonuclease Cas9,5 effect sequence-specific DNA cleavage in a genome. This programmable cleavage can result in mutation of the DNA at the cleavage site via non-homologous end joining (NHEJ) or replacement of the DNA surrounding the cleavage site via homology-directed repair (HDR).6,7 
One drawback to the current technologies is that both NHEJ and HDR are stochastic processes that typically result in modest gene editing efficiencies as well as unwanted gene alterations that can compete with the desired alteration.8 Since many genetic diseases in principle can be treated by effecting a specific nucleotide change at a specific location in the genome (for example, a C to T change in a specific codon of a gene associated with a disease),9 the development of a programmable way to achieve such precision gene editing would represent both a powerful new research tool, as well as a potential new approach to gene editing-based human therapeutics.